The present invention relates to a method for making pure metal halides useful for manufacturing metal halide glasses or the like. More particularly, the invention relates to a method for making metal halide precursors by a vapor phase deposition process, and the use of those precursors to make glasses and glass articles such as optical waveguide preforms.
Metal halide compositions are presently of interest in connection with the development of new, transparent glassy materials exhibiting unique optical properties. For example, in the field of glass optical waveguides, which are transparent glass filaments used to transmit light signals for communication purposes, glasses of improved transparency are continually being sought.
Commercial glass optical waveguides are presently formed of oxide glass materials, typically fused silica and doped fused silica glasses. Improvements in such materials have continued to the point where loss coefficients near the theoretical minimum of 0.1 dB/km at 1.6 microns have been achieved. However, it has been recognized that even lower attenuations, perhaps as low as 0.001 dB/km, might be attainable in halide glass systems which can operate at wavelengths further into the infrared region. Among the halide glasses which have been considered for the manufacture of extremely low loss optical waveguide fibers are glasses based on BeF.sub.2, ZrF.sub.4 and ZnCl.sub.2.
Examples of patents disclosing the use of metal halide glasses for infrared transmitting optical devices are U.S. Pats. Nos. 4,189,208, 4,308,066, and 4,343,638. These patents, however, teach preform or filament manufacture by conventional batch melting and forming methods.
Vapor deposition methods for making pure metal halides for metal halide glass optical devices have been proposed in U.S. Pat. No. 4,378,987 and in published Japanese Patent Application No. 57-051146. These methods offer the potential of providing the purity necessary for very low attenuation optical devices.
In the method of U.S. Pat. No. 4,378,987, a particulate metal halide precursor such as a metal fluoride powder is generated by reacting a vaporous metal source such as an organometallic compound (e.g., a metal alkyl or a metal beta-diketonate) with a vaporous halogen source such as HF. These reactive vapors are flowed into a reaction zone adjacent the substrate where they react to form the precursor, and this precursor is deposited on a substrate where it can be further processed by consolidation to form a transparent preform or fiber.
Although this method does avoid the inclusion of crucible impurities or the like in the glass, the handling of halogenating agents such as HF and HCl is difficult. HF, in particular, is corrosive and difficult to meter precisely, and if present in excess in the reaction mixture presents effluent handling difficulties. Also, due to the reactive nature of the starting materials used in this process, separate delivery systems for each of the reactants are required.
It is therefore a principal object of the present invention to provide a simplified process for making transparent metal fluoride-containing glasses by a vapor phase method, wherein the above described difficulties may be largely avoided.
Other objects and advantages of the invention will become apparent from the following description thereof.